Field of the Invention
The invention lies in the telecommunications field. More specifically, the invention relates to a method and a configuration for encoding symbols for transmission via a radio interface of a radio communications system, in particular of a mobile radio system or wireless subscriber access system.
In radio communications systems, wanted information comprising a number of symbols, for example voice, video information or other data, is transmitted by means of electromagnetic waves via a radio interface between a transmitting and a receiving radio station. The electromagnetic waves are thereby transmitted using carrier frequencies within the frequency band intended for the respective system. In the GSM mobile radio system (Global System for Mobile Communications)xe2x80x94see, for instance, Eberspxc3xa4cher and Vxc3x6gel xe2x80x9cGSM Global System for Mobile Communicationxe2x80x9d, B. G. Teubner, 1997xe2x80x94the carrier frequencies are in the range of 900 MHz, 1800 MHz and 1900 MHz. For future radio communications systems, for example the UMTS (Universal Mobile Telecommunication System) or other 3rd generation systems, the intended frequencies are in the frequency band around 2000 MHz. Frequency-division multiplexing (FDMAxe2x80x94frequency division multiple access), time-division multiplexing (TDMAxe2x80x94time division multiple access) and/or code-division multiplexing (CDMAxe2x80x94code division multiple access) methods as well as combinations of these known methods are used to distinguish between different signal sources at the location of the respective receiver.
In order to ensure adequate transmission quality (which is characterized, for example, by a low bit error rate), error protection encoding is added at the transmitter end to the wanted information for transmission of the wanted information via the radio interface. The error protection encoding allows reconstruction, at the receiver end, of faulty symbols that occur as a result of interference during the transmission via the radio interface. Particularly in the UMTS mobile system, which is optimized for data transmission at high data rates, for example for multimedia applications, high transmission quality must be achieved for effective utilization of the limited radio resources available. In this case, for example, a bit error rate of 10xe2x88x926 is required for data transmission, while a bit error rate of 10xe2x88x923 is sufficient for voice transmission.
One known method for error protection encoding, which is also called forward error correction (FEC), is, for example, convolution coding which is used, inter alia, in the GSM mobile radio system. A specific number of faulty symbols can in each case be identified and corrected at the receiver end depending on the convolution coding rate, that is to say the ratio between an original number of symbols and the number of coded symbols resulting from the addition of redundancy.
In addition to convolution coding, the symbols cain be block-coded, for example using a so-called cyclic redundancy check (CRC). A block code consisting of parity checking symbols is thereby calculated and added to the symbols. This block code makes it possible to identify that faulty symbols have occurred during the transmission via the radio interface. However, it is not always possible to correct these faulty symbols. For example, in non-realtime services, the block code can be used to control a repeated request for faulty data packets using a known automatic request (ARQ) method, while convolution coding is mainly used for real-time services, such as for voice transmission.
Alternatively, channel encoding may likewise be carried out by means of turbo encoding. That process is known, inter alia, from C. Berrou et al., xe2x80x9cNear Optimum Error Correcting encoding and Decoding: Turbo-Codesxe2x80x9d, IEEE Transactions on Communications, Vol. 44, No. 10, Oct. 10, 1996, pages 1261-71. In turbo encoding, very long codes are produced at the transmitter end by parallel connection of two convolutional coders as well as a special turbo code interleaver. These very long codes are then decoded at the receiver end by iterative decoding of the component codes.
As an alternative to turbo encoding, chains of a plurality of component codes are also used to produce long codes to form an overall code. This is known, for example, from M. Bossert, xe2x80x9cKanalcodierungxe2x80x9d [Channel encoding], 2nd ed., B. G. Teubner Verlag, Stuttgart, 1998, page 325. The component codes are threby distinguished on the basis of inner and outer codes. Convolution coding and Reed-Solomon encoding in conjunction with interleaving are used, for example, as encoding schemes for the inner and outer codes.
The prior art error protection encoding processes (apart from turbo encoding) have a common feature in that, although the bit error rate advantageously falls exponentially with the length of the codes used, and the transmission quality rises, the decoding complexity rises exponentially with the length of the codes, however, in a disadvantageous way. On the other hand, turbo encoding has already been optimized with respect to a low level of decoding complexity, but a low bit error rate of approximately 10xe2x88x926, which is required for data transmission, can be achieved only if the signal-to-noise ratio is high, that is to say in very good reception conditions, owing to the specific intrinsic types of turbo encoding.
It is accordingly an object of the invention to provide a method and a configuration for encoding symbols for transmission via a radio interface of a radio communications system, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allow good error protection, corresponding to the described field of application, with a low level of decoding complexity.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of encoding symbols for transmission via a radio interface of a radio communications system, which comprises:
splitting a terminated input string with symbols with timing control into at least two input sequences;
individually encoding the input sequences with an outer code and forming output sequences;
combining the output sequences symbol-by-symbol into an output string with symbols; and
encoding the output string with an inner code.
In other words, a terminated input string with symbols to be transmitted via a radio interface of a radio communications system is split, with timing control, into at least two input sequences. The input sequences are each individually coded by means of an outer code to form output sequences, and the output sequences are combined symbol-by-symbol into an output string with symbols. The output string is then coded by means of an inner code.
The functionality of the method according to the invention can be described in that the input strings are written line-by-line into a matrix where they are provided line-by-line with a respective outer encoding, are read column-by-column on the basis of the outer encoding and are combined into the output string, which is then provided with an additional inner encoding. The chaining according to the invention of the outer and inner encoding advantageously allows long codes to be used to ensure a low bit error rate, with a low level of decoding complexity at the same time.
In accordance with an added feature of the invention, the input sequences are coded using an outer code which is in each case different. The different outer codes advantageously make it possible to distinguish between the output sequences, and allow the transmission quality to be increased further.
In accordance with two additional features of the invention, convolution coding or block coding is respectively carried out as the outer or inner encoding. Alternatively, the inner encoding can also be carried out by means of modulation in accordance with a modulation scheme. When convolution coding is used for both the outer and inner encoding in conjunction with iterative decoding, it is possible to achieve bit error rate results which are less than the bit error rate of known turbo encoding.
In accordance with a further development of the invention, a number of symbols, which corresponds to the number of input sequences, in the output string is in each case coded by means of the inner code. In this way, the coded symbols in each column are provided with the inner encoding in accordance with the matrix representation.
According to further alternative developments, the symbols in the respective output sequences, the symbols in the output sequences or a number of symbols in the output string are in each case additionally interleaved. In the additional interleaving process, directly successive symbols are scrambled in such a way that they are no longer successive at the output of the interleaving device these developments advantageously allow the interference immunity to be improved, in comparison with the transmission of groups of faulty symbols, which frequently occur, via the radio interface.
In the first alternative development, the symbols in the respective output sequences can additionally be interleaved in accordance with individual interleaving algorithms.
In the second alternative development, the interleaving can in contrast be carried out symbol-by-symbol, that is to say the symbols in a respective column are interleaved, or on the basis of the output sequences or of a respective part of the output sequences, that is to say the symbols in the entire output sequences are interleaved.
In the third alternative development, a number of symbols, corresponding to the number of input sequences, in the output string can additionally be interleaved, by which means one column is in each case interleaved in accordance with the matrix representation.
With the above and other objects in view there is also provided, in accordance with the invention, a configuration for encoding symbols for transmission via a radio interface of a radio communications system, comprising:
a splitting device with timing control for splitting a terminated input string with symbols into at least two input sequences;
a plurality of outer encoding devices connected downstream of the splitting device in a direction of signal flow, the outer encoding devices individually encoding the input sequences to form output sequences with an outer code;
a combination device connected downstream of the outer encoding devices in the signal flow direction, the combination device combining the output sequences symbol-by-symbol to form a terminated output string with symbols; and
an inner encoding device connected downstream of the combination device in the signal flow direction, the inner encoding device encoding the output string with an inner code.
In accordance with again an added feature of the invention, there is provided at least one interleaving device for interleaving the symbols in the respective output sequences each connected downstream of a respective one of the outer encoding devices.
In accordance with again another feature of the invention, at least one interleaving device for interleaving the symbols in the output sequences is connected downstream of the outer encoding devices.
In accordance with again a further feature of the invention, at least one interleaving device for respectively interleaving a number of symbols in the output string is connected downstream of the combination device.
The above-summarized configuration is utilized, in accordance with a concomitant feature of the invention, in a transmitting device of a mobile station and/or a base station of a mobile station of a mobile radio system.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and configuration for encoding symbols for transmission via a radio interface of a radio communications system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.